1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly relates to a semiconductor integrated circuit having a termination resistor circuit.
2. Description of the Related Art
In association with the advancement of a semiconductor technology, a large-scaled LSI with a large number of input/output terminals has been manufactured. A manufactured LSI is tested by using an LSI tester in order to check whether the LSI operates normally. The LSI tester has test terminals, to which the input/output terminals of the LSI are connected.
Typically, the input/output terminals of the LSI include termination resistors. In particular, in the recent LSI, the termination resistors are employed on a chip, as shown in DDR-2 SDRAM Technology and Design Trends (DesignCon 2003). FIG. 1 is a circuit diagram showing the configuration of an input/output circuit configured in accordance with a DDR-2 standard. The input/output circuit has two sets of Thevenin termination resistors, which are low in resistance and different in resistor value (for example, 75Ω and 100Ω), on the basis of the characteristic impedance of a propagation line.
In the input/output circuit shown in FIG. 1, before the LSI test is started, a switch SW1 and a switch SW2 are opened. When the number of the input/output terminals of the LSI as a test target is small, all of the input/output terminals of the LSI can be connected to the test terminals. However, when the test target is the large-scaled LSI, there is a case where the number of the input/output terminals of the large-scaled LSI exceeds the number of the test terminals of the LSI tester. In this case, there are the input/output terminals which cannot be connected to the test terminals and cannot be controlled by the LSI tester. Therefore, it is necessary to carry out the LSI test over a plurality of times for all of the input/output terminals. The input/output terminals that are not connected to the test terminals are unstable in a signal level, when an input/output circuit is tested in an input mode. If any one of the Thevenin termination resistors of the set, e.g., a resistor Rvall on a Vss side in FIG. 1 is connected to the ground line in order to stabilize the signal level of the input/output terminal in the input mode, a large quantity of penetration current flows through that resistor of the input/output circuit in an output mode so that an consumption current is largely increased.
In order to eliminate the input/output terminals that are not connected to the test terminals on the test of the large-scaled LSI, the LSI tester must be prepared which has the test terminals for the number of the input/output terminals of the large-scaled LSI. However, the preparation of such an LSI tester requires a huge cost.
Therefore, a technique is demanded which can stabilize the signal levels of the input/output terminals in the input mode and decrease the penetration current in the output mode, even if there is any input/output terminal that is not connected to the test terminals of the LSI tester, when the LSI test is carried out.